Multiple questions pertaining to the same topic. We know the Sirians landed in ancient Epypt, and went extinct some time before 1478 BC (TFE's setting) during/shortly after the sacking of Thebes.

However, in TSE we are told the Sirians lost a scout ship around 600 AD over Sierra de Chiapas... and they had contact with the wizards well in the middle ages, 1100 AD. This is thousands of years after they supposedly went extinct.

Also, we were lead to believe the Time-Lock was unique (time travel is hard), and even the Sirians didn't really know how it worked (it was build by the even older Eillian race). But somehow the Sirians could happily hand out "portals" to the Mayans and the Babylonians, that not only teleport you through space, but also time... and without the "only one use" limitation of the Time-Lock. Huh? How did they get this tech? How did they have contact with civilizations long after they were extinct?

It can be argued that ancient Mayan empires existed even that far back, we just didn't know about it, but it would be a stretch even then. Babylon on the other hand can work perfectly. But Medieval Europe will definitely a problem to explain without any sort of "time traveling".

I think the best would be for CT to eventually retcon TSE in a way that it makes more sense.

I know TSE is silly... but do you really want to retcon the game that's most people's favorite in the series?
Also, let's be honest for a second. Aside from us, are there THAT many people who care enough about Serious Sam's plot to notice these things?

Yes, and that's why it's so "easy". TSE's story can be changed to make more sense without ever touching the gameplay. So those who don't care can continue not caring, and those who do, they will get a story that makes more sense.

I agree, giving TSE a small rewrite in order to fit with the more story and lore focus approach would be cool, as long as it doesn't lose the humor of the messages (like the lions and the convicts).

__________________"Is there any better Gerüstbaufirmenvorarbeiterschlüsselanhängerfarbenwasserlöslichkeitskontrollchemiker than discy? No!" -Shrinker
[22:30] Fiendian: Discy, Finzy and Louva shall merge into a superentity known as the Lickyfonz.
"Can we just move Discy's posts to the HOF automatically already? :P" -Jetty

Watch out Sam, this chamber was used for punishing infidels, spies and criminals. The condemned were thrown from the ledges high above into this chamber, filled with lions. The lions immediately knew what to do with each convict. And the convict knew that the lions knew what to do with him. And the lions knew that the convict knew what lions will do to convicts. And convict... well that's enough.

Find your way out of here quick, I don't like it very much, it gives me the creeps all over my circuits.

This.

__________________"Is there any better Gerüstbaufirmenvorarbeiterschlüsselanhängerfarbenwasserlöslichkeitskontrollchemiker than discy? No!" -Shrinker
[22:30] Fiendian: Discy, Finzy and Louva shall merge into a superentity known as the Lickyfonz.
"Can we just move Discy's posts to the HOF automatically already? :P" -Jetty

We really need a somewhat kinda like Extended version of this game that every game's lore make sense.
Though, I still sense something that the "Earthlings" somehow discovered these Artifacts, and they decided to hid them. This means that the people found everything from these Sirians. All the things related to them crashing on South America, and engaging with the Wizards might be before they were extinct. I still think the Wizards existed long ago like them. Then comes Serious Sam....

I'm guessing the Hum-Tah were the ones who created the Timelock in case of something BIG goes wrong like Mental's war against other species.

Perhaps so, but judging by Mental's actions in SS3, it looks like even he doesn't know what to do with it so he had to resort to destroying the planet. If it was Hum-Tah tech, I think he'd try to acquire it and use it for his own ends since he's a Hum-Tah himself.

Also, Mental doesn't exactly win the war even if Sam didn't get into the Timelock in time. Sure, he wiped out all the intelligent life in the 22nd century, but he lost all of his generals (assuming we fight some of his other ones with SS4 and SS5 like Achriman) and Sam nearly killed everything he had.